This invention relates to a method of manufacturing printed wiring boards with selective solder coating on circuit pads and holes, and more particularly to an improved method of obtaining a printed wiring board having the characteristics of one with a solder mask over bare copper.
A printed wiring board (PWB) or printed circuit board (PCB) is employed as a low cost vehicle for interconnecting various components in an electrical circuit. A simple printed wiring board can be made by applying a resist material to a copper foil that is cladded to a fiberglass core so as to define circuit patterns and chemically etching away unwanted copper foil. The particular method of applying the resist, the type of resist, and the type of etchant are process details that are dictated by the end requirements of the printed wiring board and the facilities that are available. In the manufacture of double sided PC boards, holes are drilled through the board and copper is plated into the holes for interconnecting copper foil on opposite sides of the board prior to forming circuit patterns in the foil. Circuit components are located on the boards, with leads extending through associated holes, prior to a mass soldering operation which may be wave soldering of the circuit board. The most commonly used solder is a tin-lead eutectic alloy of 63% tin and 37% lead which has a relatively sharp melting point of around 361.degree. F. The operating temperature for associated wave soldering is somewhere in the range of 425.degree.-525.degree. F., with 490.degree. F. being optimum. Poor solderability can take place at lower temperatures. Higher soldering temperatures can damage heat sensitive components, cause board warpage and/or cause excessive oxidation of molten solder. During wave soldering, undesirable side affects that may occur are excessive solder consumption, which increases the weight of the wiring board when large ground plane areas thereof become coated with solder, and a tendency for solder to bridge between circuit pads and traces as well as between adjacent circuit traces. In order to reduce the amount of solder that is applied to circuit areas such as ground planes and traces and the tendency for solder bridging, and to protect electrical circuitry from contamination, a solder mask or solder resist is often applied over bare copper circuit traces and bare ground planes. This resist is an organic coating that is applied over the entire board, except for windows around areas where a solder joint is required. This mask works well over bare copper traces and ground planes.
In order to improve solderability of circuit pads and holes, the practice is to precoat them with a solderable metal. The most desirable material for this application is an alloy of tin and lead which is similar to the eutectic alloy that is actually used in the subsequent mass-wave soldering operation. The tin-lead coating is generally electroplated onto the pads and in the holes and then reflowed. It may also be applied as a solder dip coating. Although other electro-deposited metals such as tin, nickel, tin-nickel alloy and even gold may be used to cover the copper foil-traces, solder plate is preferred due to its compatibility with the solder material used in wave soldering and the fact that it will melt during wave soldering and cause a liquid to liquid contact. Tin is next in preference with a melting point of only 450.degree. F. Tin-nickel does not solder well and has a very high melting point which is much greater than that of tin-lead. Gold is cost prohibitive and a severe contaminant in the wave solder process, as well as causing brittle solder joints.
In the past, when a tin-lead solder overplating was used as an etch resist for the copper traces and ground planes and had a solder mask applied over it, the solder under the mask would melt and flow sufficiently during wave soldering to cause bridging of circuit traces under the solder mask as well as wrinkling and/or rupture of the solder mask itself. This problem is described in ones of the references "Solder Coating and Leveling Competes With Tin-Lead Electroplate" by H. W. Markstein, ELECTRONIC PACKAGING AND PRODUCTION, December 1982, pages 30-35; "Selective Solder Coating; Solving Your Customer's Problems" by C. R. Smith, PRINTED CIRCUIT FABRICATION, Vol. 4, No. 8, August 1981; "Processing Using Solder Mask Over Tin-Nickel" by E. F. Duffek, ELECTRONIC PACKAGING AND PRODUCTION, June 1979, starting on page 71; and U.S. Pat. No. 4,104,111, issued Aug. 1, 1978, "Process for Manufacturing Printed Circuit Boards" by R. L. Mack; which are incorporated herein by reference. This wrinkling is unattractive and subjects the mask to peeling and cracking which allows acid based flux to attack the circuit patterns and reduces the mask's affect as a conformal coating that is employed to protect the circuit from the elements, e.g., high humidity and corrosive atmospheres. The basic problem then is to produce a circuit board that solders well and has a solder mask that will not wrinkle during a wave solder operation.
One approach is to apply the solder mask over bare copper traces and ground planes. Another prior art technique is to chemically remove any tin-lead solder overplating from copper traces and ground planes prior to application of the solder mask, as is described in the Smith and Markstein articles. This adds the cost of an additional removal operation to the manufacture of the circuit board as well as the problem of removing lead from waste products. Another prior art technique is to overplate the copper of the circuit patterns with a high melting point metal, i.e., a tin-nickel alloy, and then selectively plate tin-lead solder on only the areas of holes and pads, the solder mask being applied over the tin-nickel on the traces and ground planes. Since the melting point of the tin-nickel overplate is much greater than that of the molten solder in the wave soldering operation, the tin-nickel does not melt and there is no deformation of the solder mask. This prior art technique is well described in the patent and Duffek article. Unfortunately, this prior art technique burdens a PC board manufacturer with the expense and Environmental Protection Agency requirements of having an additional plating bath for tin-nickel in his facility and his employing an additional step in the manufacturing operation, something that PC board manufacturers have been reluctant to do, as is documented in the Smith article.
An object of this invention is the provision of an improved method of manufacturing printed wiring boards.